All along it was clear that the diplomatic effort would fail, because the United States refused to use military pressure against the regime of Bashar al-Assad — a lack of leverage that Mr. Kerry himself lamented in a meeting with Syrian activists. Yet the pleading phone calls to Moscow continued, day after day, and consideration of other U.S. options in Syria remained on hold, even as eastern Aleppo was reduced to rubble.

Administration supporters ruefully wondered why Mr. Putin chose not to go forward with a cease-fire plan that was, as former State Department and National Security Council official Philip Gordon put it, “a clean win” for Russia. The deal negotiated by Mr. Kerry would have left the Assad regime in power indefinitely while Russia and the United States joined in a military campaign against its opponents — those rebels deemed to be “terrorists.”

The simple answer is that, having pocketed those U.S. concessions, Mr. Putin chose to pursue a still more decisive victory, the recapture by the regime of Syria’s largest city. If the offensive succeeds, any possibility of Mr. Assad leaving power will be removed. If it fails, the Kremlin reckons it can always go back to the ever-willing Mr. Kerry.

Mr. Putin clearly calculates that he has nothing to fear from the United States; so confident has he become that he canceled a nuclear cooperation agreement with Washington on Monday and decreed a list of stiff conditions for restoring relations, including the lifting of sanctions and compensation to Russia for their costs. The White House, meanwhile, still has not responded to Russia’s hacking of the Democratic National Committee or even publicly acknowledged Moscow’s role.

The administration is going through the motions of considering new options in Syria. According to The Post’s Josh Rogin, possibilities to be weighed in a Cabinet-level meeting Wednesday include cruise-missile strikes to ground the Syrian air force and the supply of more advanced weapons to the rebels defending Aleppo. Senior Pentagon and CIA officials are said to support those steps, and for good reason: Their assessment is that the fall of Aleppo would worsen the terrorist threat from Syria.

President Obama has rejected military options numerous times on the grounds that they would only exacerbate the conflict. The result has been a steady expansion of Syria’s carnage, the growth of terrorist forces and the shrinking of U.S. influence, to Russia’s gain. Continuing inaction will compound those disastrous results.